Recently, environmental protection and energy conservation have received widespread attention from all sectors of society. In the lighting field, people's environmental protection and energy conservation awareness is expressed as mass use of LED lighting products having particular energy-saving advantages.
In prior arts, driving circuits for LED lighting products generally employ a conventional switch-mode constant current power supply technology (AC-DC). The lifetimes of these driving circuits are far shorter than the lifetimes of the LED themselves. This leads to that the LED lighting products are inferior to the conventional energy-saving lamps in terms of real lifetime. Thus, there is proposed a first-generation AC LED driving technology in which LEDs are directly input with AC mains supply. This technology is capable of solving the lifetime problem in conventional driving power supplies, but has a very low driving efficiency, usually lower than 70%, while the switch-mode constant current power supply usually has a driving efficiency of greater than 80%. Further, in the first-generation AC LED driving technology, the working currents of LEDs frequently change, and a surge occurs in each cycle of the AC mains supply, which is bad for the lifetimes of LEDs and meanwhile causes reduced light emitting efficiency of LEDs and significant flicker viewed by people's eyes with the fluctuations of the mains supply.
Aiming at the above defects of the first-generation AC LED driving technology, there is proposed a second-generation AC LED driving technology. In this technology, previous single LED string is divided into a plurality of units each of which is formed by one or more LEDs connected in series, and a ground switch is provided for each unit, and then a current-limiting device is provided. During operation, a control circuit detects input voltage values, and selects one of switches to be turned on according to the voltage values. The advantages of the driving technology are as follows. The AC mains supply presents a sinusoidal waveform in which voltage changes over time. When the voltage is relatively low, the first unit is powered on; when the voltage rises, two units, i.e., the first and second units, are powered on; when the voltage starts to fall down, the first unit is powered on again, and so on. Thus, the driving efficiency is increased from previous lower than 70% to 90%. Meanwhile, because current-limiting technology is used, a peak value of the current flowing through LEDs is limited, and thus LEDs are protected, and thereby the problem of significant flicker viewed by people's eyes with the fluctuations of the AC mains supply is solved.
Although the second-generation AC LED driving technology solves the problems with the first-generation technology, the second-generation technology still has some shortcomings. For example, according to the present market demands, it is desired that LED lighting products can be compatible with conventional TRIAC dimmer to realize a dimming function. It is hard for conventional switch-mode constant current power supply to satisfy such demands, because the dimmable switch-mode constant current power supplies available on the market have bad dimming effects (flicker is generated). Also, the first-generation AC LED driving technology cannot meet the demands because there are dramatic sudden changes in brightness (sudden brightening or darkening). For the second-generation AC LED driving technology, there is no dramatic flicker and sudden change in the brightness during dimming, and however a phenomenon where LEDs in a lot of units are not powered on occurs during the dimming, and this results in that a part of the light outgoing surface do not have light. As a result, although brightness adjustment is realized, light output is influenced.
Further, in the second-generation AC LED driving technology, the number of working LEDs varies in different time periods, and all the LEDs emit light only in very little time. As a result, the LEDs are not fully utilized.
Further, even though the second-generation AC LED driving technology employs the current-limiting technology, the technology does not address the problem of the output light brightness of LED lighting products. That is to say, depending on how many units are divided into, there will be brightness stages the number of which is double of the number of the divided units (the brightness is constant within a certain time period, and when the voltage rises, the brightness will rise accordingly to a certain value and then remain at this value).
Further, same as the first-generation technology, if using the second-generation AC LED driving technology, one type of product can work under only one grid voltage. For example, if an AC product of 110V works in an electrical grid of 220V, the brightness and power of the LED product will increase and even get damaged. Similarly, if a product of 220V works in a grid of 110V, the power and brightness of the product will be reduced and the product may even become dark.
Thus, it is desired to develop an AC LED driving circuit which is capable of accomplishing the advantages of the second-generation AC technology, and meanwhile keeping constant light output of LEDs without brightness stages. Further, it is also desired that the AC LED driving circuit can work under different grid voltages as conventional switch-mode constant power supplies and remain at constant power and brightness.